Certain vehicles such as cruise missiles, interceptors, re-entry vehicles and high-speed aircraft may operate in the supersonic and hypersonic flight regimes. Such vehicles must be capable of withstanding significant heat loads caused by aero-thermal heating of the outer surface of the vehicle. For example, the nose tip of a missile flying at hypersonic speeds at low altitude can reach stagnation temperatures exceeding the melting point of tungsten (approximately 6,000° F.). Such heating can result in material ablation which can alter the shape of the nose affecting the aerodynamics and controllability of the missile.
For certain hypersonic vehicles such as missile interceptors, an optical sensor for target acquisition may be located at the nose of the vehicle and is preferably oriented in a forward-facing direction for optimal signal transmission. The sensor is typically covered by a sensor window which must be capable of withstanding the extreme heat environment at the nose tip. For example, the sensor window may be formed of sapphire due to its favorable optical and mechanical properties at elevated temperatures.
Optical signals from the optical sensor must pass through a bow shock wave which typically forms at a location forward of a missile or other blunt-nosed object in supersonic or hypersonic flow. The bow shock is typically detached from the object and at lease partially envelopes the nose section.
One prior art mechanism for regulating the temperature of the sensor window is by actively cooling the window with a thin film of fluid. However, such cooling systems require high pressure purge gas and associated plumbing as well as an activation system, all of which add complexity and weight to the vehicle. Furthermore, the thin film of fluid on the sensor window may affect optical signal quality.
Another approach to reducing the temperature of the sensor window is to relocate the window from the forward-most point on the nose tip to a relatively lower temperature area along the side of the nose. Although the heating environment may be more favorable, the quality of optical signal transmission may be adversely affected. For example, as compared to optical signals transmitted from a centrally-located window at the nose tip where the signals pass through the bow shock at a perpendicular angle, optical signals from a side-located window must travel through the bow shock layer at an oblique angle which may reduce signal quality.
Another approach to reducing the temperature of the sensor window is to locate the window at the base of a forward-facing cavity formed in the nose tip. Placement of the optical sensor window at the basewall of the cavity has been shown to be an effective means for reducing heat transfer as compared to heat transfer at a sensor window integrated into a forward-most location of a conventional nose. For example, the heat flux measured at the cavity basewall of a forward-facing cavity may be an order of magnitude less than the heat flux measured at the stagnation point of a conventional convex nose tip.
However, one characteristic of forward-facing cavities in supersonic or hypersonic flow are oscillations in pressure that occur within the cavity. The pressure oscillations are driven by cavity geometry and can affect vehicle performance and optical signal quality. For example, such pressure oscillations in the cavity can cause an increase in heating at the cavity basewall as compared to a cavity with non-oscillating pressure. The frequency of such pressure oscillations has been found to closely correspond to the organ-pipe frequency associated with resonance tube theory wherein the frequency is a function of cavity depth.
A further characteristic associated with cavity pressure oscillations are oscillations that are induced in the bow shock. The cavity-driven bow shock oscillations occur at relatively high amplitudes resulting in large fluctuations in aerodynamic drag of the vehicle. In this regard, bow shock oscillations complicate vehicle control and interfere with optical signal transmission which may compromise target tracking.
Attempts to reduce or dampen the amplitude of such bow shock oscillations include the injection of pressurized gas such as helium into the cavity in an attempt to stabilize the cavity pressure fluctuations. Attempts to dampen bow shock oscillations also include the application of pulsed energy to the cavity such as by using laser energy in order to stabilize the pressure fluctuations. However, such systems require additional hardware which adds to vehicle complexity and weight.
As can be seen, there exists a need in the art for a system and method for damping pressure oscillations occurring within a cavity in order to minimize heating of a sensor window at the cavity basewall. Furthermore, there exists a need in the art for a system and method for reducing bow shock oscillations in order to minimize fluctuations in vehicle drag and improve vehicle controllability. Ideally, such a damping system is simple in construction and low in cost.